The present invention relates to a method of manufacturing a solid electrolytic capacitor and, more particularly, to a method of manufacturing a solid electrolytic capacitor using a conductive polymer compound as a solid electrolyte.
A general solid electrolytic capacitor has a structure in which a molded porous body consisting of a valve metal such as tantalum or aluminum is used as an anode, an oxide coating of the valve metal is used as a dielectric, and a solid electrolyte such as manganese dioxide or a 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex salt serves as part of a cathode. In this case, although the solid electrolyte has a function of electrically connecting the entire surface of the dielectric in the molded porous body to an electrode lead, the solid electrolyte desirably has a function of repairing electrical short-circuiting caused by the dielectric oxide coating defect. For this reason, a metal having a high electrical conductivity but having no dielectric repair function is not suitable as the solid electrolyte. Therefore, manganese dioxide or the like which is changed into an insulator due to heat generated by a short-circuiting current has been used as the solid electrolyte.
However, when manganese dioxide is used as part of an electrode, the impedance is high in a high-frequency range because the conductivity of the manganese dioxide is not satisfactorily high. On the other hand, when the TCNQ complex salt is used as part of an electrode, heat resistance is lowered because the TCNQ complex salt tends to be thermally decomposed. Therefore, solid electrolytic capacitors using the above materials have various problems left unsolved.
In recent years, new solid electrolyte materials have been developed in the field of polymers. As a result, various solid electrolytic capacitors using, as solid electrolytes, conductive polymer compounds obtained by doping electron donor or acceptor compounds (dopants) in conjugated polymer compounds such as polypyrrole, polythiophene, and polyaniline are proposed. For example, a method of forming a polyaniline film by polymerizing an aniline monomer on a dielectric oxide coating using an oxidant is proposed. However, according to this method, only a polyaniline film having a small thickness can be formed. For this reason, this polyaniline film may be removed from the dielectric oxide coating by thermal stress generated when a solid electrolytic capacitor is mounted. At this time, the oxide coating may be damaged, so that an increase in leakage current of the capacitor may disadvantageously occur.
In contrast to this method, a solid electrolytic capacitor manufactured by the following method is disclosed in Japanese Patent Laid-Open No. 3-35516 (Reference 1). That is, polymerized polyaniline soluble in an organic solvent is coated on a dielectric oxide coating, dried, and then doped with proton acid to make the polyaniline conductive. According to this method, a polyaniline film having a large thickness can be formed. However, the polyaniline solution used in the method cannot satisfactorily permeate the entire fine uneven surface of the dielectric oxide coating because the polyaniline solution has a very high viscosity. Therefore, only a capacitor having a considerably low capacitance reproduction ratio (i.e., the ratio of an actual electrostatic capacitance value to a design value) can be disadvantageously manufactured.
As described above, a solid electrolytic capacitor in which a conductive polymer compound obtained by polymerization on a dielectric oxide coating using an oxidant is used as a solid electrolyte has a thin solid electrolyte layer. Therefore, the dielectric oxide coating is damaged by thermal stress generated upon mounting the solid electrolytic capacitor, thereby disadvantageously increasing a leakage current.
In a solid electrolytic capacitor in which a polyaniline film made conductive by coating, on a dielectric oxide coating, polymerized polyaniline soluble in an organic solvent, drying the polymerized polyaniline, and doping proton acid in the polymerized polyaniline is used as a solid electrolyte, a polyaniline solution cannot permeate the entire fine uneven surface of the dielectric oxide coating because the polyaniline solution has a very high viscosity. Therefore, only a capacitor having a very low capacitance reproduction ratio can be disadvantageously manufactured. A method of manufacturing a solid electrolytic capacitor which simultaneously solves the above-described two problems has not been developed.